1. Field of the Invention
The present invention relates to an optical sensor (optical fiber sensor) detecting physical quantities such as pressure and temperature by measuring a variation in light intensity, and more particularly, to an optical sensor which can maintain high measurement precision even when a light transmission path is distorted, by suppressing polarization dependency with a low-cost and simple device configuration.
Priority is claimed on Japanese Patent Application No. 2007-145012, filed May 31, 2007, and Japanese Patent Application No. 2007-219672, filed Aug. 27, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
In the past, electrical sensors were widely used as a sensor measuring physical quantities such as displacement, temperature, and pressure of an object. However, since the electrical sensors require a supply of power and transmit measured signals (electrical signals) to remote locations through wires, the measurement precision thereof is deteriorated due to an influence of electromagnetic noises. On the contrary, since optical sensors using optical fiber convert measured signals into optical signals and transmit the optical signals to remote locations through optical fiber, it is possible to transmit the signals without suffering from electromagnetic noises and to measure the physical quantities with high precision due to its small measurement error (for example, see Japanese Unexamined Patent Publication Nos. H6-8724, S57-108605, H2-57909, H3-243822, and H2-49115, S63-169521, U.S. Pat. Nos. 5,068,527, 4,996,418, and 4249076, Japanese Unexamined Patent Publication Nos. H11-352158 and 2004-301769, Japanese Patent No. 3304696, and Japanese Unexamined Patent Publication Nos. 2003-214966, S61-275632, and 2005-49670).
As such physical-quantity optical sensors, sensors converting a variation in pressure into a variation in distortion using a distortion member (such as a bourdon tube) having a mechanism which is distorted by pressure and sensing the distortion by the use of a fiber Bragg grating (FBG) are known. Sensors are also known in which a reflecting plate (such as a diaphragm) displaced with pressure is opposed and fixed to an end surface of an optical fiber and the displacement is detected by measuring the light emitted from the end surface of the optical fiber by the use of the light reflected from the reflecting plate, whereby the pressure is detected from the displaced distance.
FIG. 1 is a diagram illustrating an example of a related optical pressure sensor. The optical pressure sensor includes a light source 1, a reflecting surface 4a of a reflecting plate 4, a sensing unit 3 of which the relative distance from an end surface of an optical fiber varies depending on a physical quantity such as pressure or temperature, a light-emitting first optical fiber 2 transmitting light from the light source 1 to the sensing unit 3, a light-receiving second optical fiber 5 and a light-receiving third optical fiber 6 transmitting the light reflected from the reflecting surface 4a of the sensing unit 3 to two light-receiving portions (first and second light-receiving portions 7 and 9), amplifiers 8 and 10 amplifying signals from the first and second light-receiving portions 7 and 9, and a calculation unit 11 calculating the physical quantity from a ratio of the amplified electrical signals from the first and second light-receiving portions 7 and 9. The end surfaces of the first to third optical fibers 2, 5, and 6 opposed to the reflecting surface 4a are fixed so that the longitudinal direction of the optical fibers and the normal line of the reflecting surface 4a form an angle θ. The second optical fiber 5 and the third optical fiber 6 are parallel to each other and the fixing angles of the first optical fiber 2 and the second and third optical fibers 5 and 6 are symmetrical about the normal line of the reflecting surface 4a. 
FIG. 2 is an enlarged diagram of the sensing unit 3. The second optical fiber 5 and the third optical fiber 6 for receiving light are parallel to each other. The first optical fiber 2 and the second and third optical fibers 5 and 6 are fixed to be symmetrical about the normal line of the reflecting surface 4a by an angle θ. The light emitted from the first optical fiber 2 is reflected by the reflecting surface 4a and the reflected light coupled to the second optical fiber 5 and the third optical fiber 6 are transmitted to the first and second light-receiving portions 7 and 9, respectively, whereby the light intensities P1 and P2 are measured and the light intensity ratio F(P1,P2)=(P1−P2)/(P1+P2) is calculated by the calculation unit 11. Here, since the light intensity varies depending on the relative distance between the reflecting surface 4a and the end surface of the optical fibers, the light intensity ratio F(P1,P2) varies. Accordingly, by constructing the sensing unit 3 so that it is displaced depending on physical quantities such as pressure and temperature, it is possible to detect such physical quantities.
In the optical pressure sensor, the measuring instrument can be manufactured at a low cost and it is possible to easily process the measured signals. In order to measure the light intensity, factors other than displacement of the reflecting plate, for example, a variation in light intensity of the light source or a variation in light intensity due to a transmission loss, causes a measuring error, but it is possible to enhance the measurement precision by compensating for the light intensity ratio of the light received by plural optical fibers in addition to greatly reducing the variation in transmission loss.
In the situation described above in which the optical sensor became popular, the applicant of the invention suggested an optical sensor having high measurement precision, which is disclosed in Japanese Unexamined Patent Publication No. 2007-24826.
As shown in FIG. 3, the optical sensor includes a sensing unit 3 which has an object 16 having a reflecting surface 15 and of which the relative distance from the end surface of an optical fiber varies depending on physical quantities such as pressure and temperature, a first optical fiber 2 (light-transmitting optical fiber) transmitting light from a light source, second and third optical fibers 5 and 6 (light-receiving optical fibers) transmitting the light reflected by the reflecting surface 15 of the sensing unit 3 to light-receiving portions 18A and 18B, respectively, and a calculation unit 11 acquiring a ratio of electrical signals from the light-receiving portions 18A and 18B and calculating the physical quantities.
As shown in FIG. 2, in this optical sensor, light is emitted to the object 16 disposed at the relative distance D from the end surface of the first optical fiber 2, the reflected light is received by the second and third optical fibers 5 and 6, and the displaced distance of the object is calculated. The first optical fiber 2 is disposed so that the longitudinal direction thereof and the normal line of the reflecting surface of the object 16 form an angle θ, the second and third optical fibers 5 and 6 are disposed parallel to each other so that the longitudinal direction and the normal line form the angle θ. The first optical fiber 2 and the second and third optical fibers 5 and 6 are opposed to each other with the normal line interposed therebetween, and the first to third optical fibers 2, 5, and 6 have a single mode in the wavelength of which the fibers are used. Hereinafter, the optical sensor having the above-mentioned configuration is called a 3-core array sensor.
Variation characteristics of the light intensity and the intensity ratio with the variation of the relative distance D will be now described with reference to FIG. 4. In the figure, the horizontal axis represents the relative distance D, the left vertical axis represents the light intensity, and the right vertical axis represents the intensity ratio. The characteristic graph illustrates variations of the reflected light intensities P1 and P2 of the second and third optical fibers and the intensity ratio F(P1,P2) with the variation of the relative distance D between the end surfaces of the first to third optical fibers and the reflecting surface. Hereinafter, the variation characteristic is called distance dependency. F(P1,P2)=(P1−P2)/(P1+P2) was used as an expression for calculating the intensity ratio F.
As can be seen from the distance dependency, the intensity ratio F(P1,P2) forms a curve having a substantially linear slope portion. The slope portion is used to measure the physical quantities. As the slope portion is closer to being linear, a correction function for converting the variation in distance into the physical quantity is simpler, whereby the calculation is facilitated and error is reduced. On the other hand, the measuring sensitivity is expressed as Δ=dF(P1,P2)/dD of the slope, where the measuring sensitivity increases as Δ increases.
FIG. 5 shows the distance dependency of the intensity ratio F measured using the 3-core array sensor having different angles (fixing angle of fibers) θ. As shown in the figure, when the fixing angles θ of the fibers increase, the peak position of P1 and P2 gets close to the reflecting surface and thus A increases. On the contrary, when the fixing angles θ of the fibers are reduced, the peak position gets apart from the reflecting surface and thus D decreases. That is, when the fixing angles θ of the fibers vary, D varies. Accordingly, it is possible to easily select the measuring sensitivity. Here, the measuring range, that is, the range of the relative distance where the linear slope portion exists in the distance dependency, has a trade-off relationship with the measuring sensitivity. Accordingly, when the measuring sensitivity increases, the measuring range narrows. On the contrary, when the measuring sensitivity decreases, the measuring range increases.
As described above, the relative distance D can be induced from the intensity ratio F. That is, even when a variation in light intensity of the light source or a variation in light intensity due to the bending loss of the first optical fiber occurs, the intensity ratio F does not vary, thereby calculating the relative distance D with high precision.
In the related optical sensor, since the reflected light is received by two fibers parallel to each other to acquire the ratio of the light intensities thereof, it was considered that the influence of a variation in the polarized state need not be considered. For example, in the above-mentioned document, when a light emitting diode (hereinafter, referred to as “LED”) is used as the light source, the polarization degree of the light source was not described, it was considered that the polarization degree does not affect the measurement precision, and no influence was recognized.
However, the inventors of the invention verified the polarization degree of the light source and the measurement precision in detail. As a result, it was first confirmed that the measurement precision decreases even with the polarization degree of the LED light source conventionally expected and it was seen that the influence increases as the fixing angles of the optical fibers increase.
In the sensors in which the light-emitting optical fiber and the light-receiving optical fiber have different structures among the sensors disclosed in the above-mentioned documents, when the polarization state of the emitted light varies, the light intensity varies, thereby reducing the measurement precision. There has been suggested a structure in which a certain specific angle is given to the light-emitting optical fiber and the light-receiving optical fiber. However, when light is reflected while an angle is given, it is more easily affected by the variation in polarization state, thereby further reducing the measurement precision.
In the sensors using a difference in wavelength between plural LEDs among the sensors disclosed in the above-mentioned documents, since the number of components increases, the device configuration is complicated and the cost increases.
A simple optical sensor (optical fiber sensor) with low cost and high precision is not realized in which the polarization dependency is suppressed in a light source having a small polarization degree like an LED.
The present invention is contrived to solve the above-mentioned problems. An object of the present invention is to provide a simple optical sensor with low cost and high precision in which the polarization dependency is suppressed.
In the above-mentioned 3-core array sensor, when the variation difference in light intensity occurs due to the bending loss in the second and third optical fibers, the intensity ratio F also varies, whereby the precision of the calculated relative distance D is reduced.
The invention is contrived to solve the above-mentioned problem. Another object of the invention is to provide an optical sensor that can suppress a decrease in measurement precision due to the bending loss in a light transmitting path.